Method of manufacturing alloys



Patented Aug. 27, 1935 PAENT OFFiC METHOD OF MANUFACTURING ALLOYS Wilhelm Rohn, Hanau-on-the-Main, Germany, assignor to Heraeus-Vacuumschmelze A. G., Ilanau-on-the-Main, Germany, a company of Germany No Drawing.

Application September 7, 1934,

scgilal No. 743,156. In Germany December 21, 19

15 Claims.

This invention relates to metallurgy and more particularly to a method of forming alloys substantially free from associated deleterious impurities of carbon, silicon, oxygen, nitrogen, sulfur, phosphorus and the like. This application is a continuation-in-part of application Serial No. 642,464, filed Nov. 12, 1932.

Heretofore in the art many methods have been devised for manufacturing alloys. In the main these methods are characterized by the fact that the resultant alloy product contains in addition to the desired alloy constituents small proportions of other elements which being unnecessary and'even deleterious are undesirable. This is particularly true of alloys wherein the base elements comprise one or more of the metals iron and nickel and the added alloy constituents comprise one or more of the metals chromium, molybdenum, tungsten, vanadium, tantalum, columbium, titanium, zirconium, uranium, copper, manganese, aluminum, magnesium and the like.

In most of these alloys residual amounts of carbon, sulfur and phosphorus are highly undesirable. It has recently been recognized also that the elements oxygen and nitrogen deleteriously affect certain chemical and physical properties of these alloys. tedious process to eliminate these residual impurities from alloys of the above identified types.

One of the objects of the present invention is to provide a method for manufacturing alloys particularly those alloys having a base consisting of at least one of the metals iron and nickel, which method provides for the substantial exclusion and elimination of deleterious residual impurities of carbon, sulfur, phosphorus, oxygen, nitrogen, hydrogen and the like. Another object of the present invention is to provide a method for manufacturing iron base alloys substantially freefrom residualimpurities of carbon, sulfur, phosphorus, oxygen, nitrogen, hydrogen and the like.

Still another object of this invention is to provide a method for manufacturing iron base alloys containing chromium substantially free from residual impurities such as carbon, sulfur, phosphorus, oxygen, nitrogen, hydrogen and the like.

Other objects and advantages will be apparent as the invention is further disclosed. I

In accordance with the objects of the present invention I have found that metal oxides may be reduced to metal by superposing the said oxide upon the surface of a molten metal bath in which the said reduced metal will dissolve, and there- In general it is an extremely and the metal of .the said bath. I have found that the rate of reduction of the said oxide is greatly accelerated by heating the bath to a relatively high temperature and by turbulently stirring the bath to obtain a substantially uniform dispersion of the metal oxide throughout the bath, thereby maintaining a substantially uniform bath composition during the progress of the oxide reduction. This turbulent stirring also accelerates the rate of reduction by increasing the efi'ective surface area of the bath and oxide exposed to the, said reducing gas.

It is apparent that the ,steps of this broad method must be specifically modified to adapt the same to the manufacture of any particular alloy, and to effectthe reduction of any particular metal oxide, and to eliminate any particular impurity from the bath.

As a specific embodiment of the practice of the present invention I will describe the method as I have adapted the same to'the manufacture of substantially carbon and oxide-free iron-chromium and iron-chromium-nickel alloys and steels. y v

I first form a molten low carbon iron bath, which is placed into a refractory lined crucible and a proportion of chromium oxide is added thereto which upon complete reduction 'will add to the alloy the desired amount of chromium. and thereafter the molten bath is heated to the desired temperature and turbulently stirred and agitated until the chromium oxide is thoroughly and uniformly dispersed the'rethrough.

The top of the crucible is then enclosed and a reducing gas is circulated over the surface of the molten metal bath. During the circulation of the reducing gas overthe metal bath the turbulent stirring of the bath is continued and the bath is maintained at a temperature favorable ,to the purposes of the present invention. In the furnace of this said patent the coil arrangement is such that the molten metal is vigorously rotated vertically in a plurality of radial planes as well as horizontally thereby recurrently bringing all parts of the bath to the surface and maintaining ,the chromium oxide thoroughly and uniformly distributed throughout the bath.

The reducing gas employed in the practice of the present invention may be varied widely without departing essentially from the nature and scope thereof depending in part upon the specific metal oxide that is added to the'molten metal bath, in part upon the specific temperature of the bath and in part upon the specific proportion of the metallic constituent'of the oxide that is desired within the metal bath.

For the reduction of chromic oxide and other metallic oxides which can be reduced only with difficulty, the metallic constituent of which is to be absorbed by a basic metal or by a basic alloy, it is essential that the work is done in an atmosphere which consists essentially of hydrogen and which does not contain any oxidizing constituents such as CO2 or water vapor (steam). It is therefore necessary to use a hydrogen which has been purified to an extreme extent from all traces of moisture, for instance, in the manner that any steam present is frozen out at temperatures of to C.

If it is desired to dispense with this freezing, one can also compensate the water vapor present n the hydrogen by the addition of hydrocarbons in such an amount as will chemically react therewith to form carbon oxide gases. For this purpose, it is empirically necessary that to 18 parts in weight of water vapor, for instance, 13 parts in weight of benzol are added. Beyond this quantity of benzol which is necessary for the compensation of the injurious water vapor, the content of benzol can be further increased, namely, up to 40 grams benzol per cubic meter of hydrogen, namely, beyond the quantity required for the compensation of the water vapor, without any undesirable carburization of the metal bath occurring thereby. In place of benzol, also other hydrocarbons can be used in equivalent quantities.

One can even go a step further and produce an atmosphere containing as its main constituent, hydrogen with certain quantities of oxygen-containing compounds and suflicient quantities of hydrocarbons to compensate the injurious action of the oxygen-containing compounds (such as water vapor), in the manner that one mixes, for instance, hydrogen with natural gas, or water gas w1th illuminating gas, or water gas with natural gas,-or water gas with poke oven gas. It appears at first sight diflicult to state the exact "quantity ratios and it appears still more difficult to main-.

tain same to a suflicient degree of accuracy in actual operation. It must be. considered as a dependable criterion for the correct composition, that the condition must prevail that just no separationof soot occurs in the furnace which is manifested by a clouding of the furnace atmosphere. For the reduction of metallic oxides such an addition of hydrocarbons is desirable, for the reason that in this way the necessityjs avoided.

of purifying the hydrogen in an extreme manner prior to the reaction.

Alternatively, I have found that the precise re- I ducing gas composition may be varied during the reduction operation with respect to the temperature of reduction employed: and with respect to the amount ofchromium in the, metal bath:

I have found that as the amount of chromium 7 iron alloy but as the amount of chromium in the alloy increases to or is in excess of about 20% the temperature of thebath to continue the reduction of the remaining chromium oxide or further additions of chromium oxide to the bath must be increased to from 1900 to 2000 C. depending upon the reducing gas atmosphere employed. The explanation of this is not at this time apparent even though recognized.

I have also found that with any given reducing gas composition the rate of reduction increases with increase in temperature. Accordingly, in view of these discoveries, I have determined that the best practice is to heat the bath to relatively high temperatures ranging from 1800" C. ,to 2000 C. even during the circulating of the initial hydrogenous reducing gas' composition thereby obtaining a relatively rapid rate of reduction of the metal oxidein the bath to a comparatively low percentage. Thereafter, on replacing this initial reducing gas with the final hydrogen atmosphere, the temperature of the bath may be increased to as close to 2000 C. as is obtainable with refractories now available.

- The presence of other alloy constituents in the molten iron bath, such as nickel, manganese,

vanadium, tungsten, molybdenum, copper and the like will not materially vary the above noted conditions. Where the alloy constituent is present in relatively small amounts the precise reducing temperatures may vary slightly. Where the other alloy constituents such as nickel or manganese is present in relatively large amounts as in .the austeni'tic iron alloys the precise reducing temperatures with any given gas and with any given per cent chromium in the bath may vary more widely from the range above indicated.

As a result of this final hydrogen treatment the molten metal bath will be found to be substantially free from residual chromium oxides and other metal oxides, and also of carbon. The.

presence of metal oxides in the molten metal bath eifectively eliminates the carbon content to a rela- It is apparent that, if desired, other metal oxides than chromium oxide may be superposed upon the molten bath, or that a mixture of metal oxides may be added in the forming of more complex alloys than iron-chromium alloys.

Under some circumstances it may be desirable to remove hydrogen from the molten metal bath at the end of the process. I have found that this may be accomplished as heretofore proposed in the art by subjecting the molten alloy at the conclusion of the final hydrogen treatment to a treatment under gradually reducing pressures whereby "the dissolved hydrogen is evolved from the molten bath and drawn oif, or by replacing the hydrogen atmosphere with an inert gas, such as nitrogen, or by a final treatment in an oxidizing duration so as not to induce material oxidation of the metal constituents of the bath, but if material oxidation occurs metallic reducing agents such as alumin alkaline earth metals may be added prior to cast--' ing the alloy to eliminate such oxide. Having hereinabove broadly and specifically disclosed the present invention and given one specific embodiment of the practice of the same, it is apparent that many modifications and departures maybe made therein without departing therefrom and all such modifications and departures are contemplated as may fall within the scope of the following claims:-

What I claim is: l. The method of'manufacturing alloys substantially free from carbon, silicon, oxygen, nitrogen, sulfur, phosphorus and the like associated impurities which comprises forming a molten bath of metal constituting the base of said alloy, superposing thereon an oxide of the metalto be alloyed with said base metal, enclosing the surface of said bath, turbulently stirring the bath to uniformly distribute the said compound throughout said bath, heating said bath to relatively high temperatures, and circulating over the surface of the highly heated and stirred bath a gaseous atmosphere reducing with respect to the said oxide, the said base element and the resulting alloy product.

2. In the method of claim 1, a reducing gas comprised in major part of hydrogen, said gas having a water vapor pressure not in excess of that remaining after cooling between '60 to -70 C.

3. In the method of claim 1, the steps of initially circulating a reducing gas comprised in major part of hydrogen and in minor part of other reducing and inert gases over said bath to reduce the major part of the oxide to metal and thereafter replacing said gas with substantially pure dry hydrogen to reduce the remainder of. said oxide.

4. In the manufacture of iron-chromium alloys in accordance with the ethod of claim 1, the step of forming the molten metal bath of a low carbon steel and incorporating therewith a proportion of other metals other than chromimum desired in the resulting alloy, and superposing thereon chromium oxide in an amount which upon complete reduction will give the desired chromium content in the alloy.

5. In the manufacture of iron-chromium alloys in accordance with the method of claim 1, the step of forming the molten metal bath bf a low carbon steel and superposing thereon a mixture ofchromium oxide and other metal oxides the metal constituent thereof is desired in the said alloy, in an amount and relativeproportion which upon give the desired chrocomplete reduction will inium and other metal bath.

6. In the method of claim comprised in major part of constituent in the said l, a reducing gas hydrogen, in minor part of'carbon monoxide and containing a proportion of an inert diluting gas, the said gas having a we. r vapor pressure not in excess of that remainingafter cooling between and -70 C. 7. In the manufacture of iron-chromium alloys in accordancewith the method of claim 1, the steps of forming the molten bath of a low carbon iron alloy, superposing chromium oxide thereon, heating the bath to temperatures at least approximating 1800 C., and circulating over the surface of the bath a reducing gas comprised in major part of hydrogen, the said gas having a uni, magnesium, alkali metals and water vapor pressure not in excess of that remaining therein after cooling to 60 to, --70 C.

8. In the manufacture of iron-chromium alloys in accordance with the method of claim 1, the steps of forming the molten bath of a low carbon iron alloy, superposing chromium oxide thereon, heating the bath to temperatures at least approximating 1800 C., and circulating over the surface of the bath a reducing gas comprised of substantially pure hydrogen, the said gas having a water 'vapor pressure not in excess of that remaining therein after cooling to -60 to 9. In the manufacture of iron-chromium alloys in accordance with the method of claim 1, the steps of forming the molten bath of a low carbon iron alloy, superposing chromium oxide thereon, heating the bath to temperatures at least approximating 1800 0., and circulating over the surface of the bath a reducing gas comprised of a mixture of hydrogen, carbon monoxide and nitrogen, the hydrogen content being predominant, the said gas being substantially free from oxidizing gases and moisture.

10. In the manufacture of iron-chromium alloys in accordance with the method ofclaim 1,' the steps of forming the molten bath of a low carbon iron alloy, superposing chromium oxide a moisture contained in said mixture.

11. The method of manufacturing iron-chromium alloys comprising forming a molten metal bath comprised of low carbon iron, superposing chromium oxide on the surface of said bath, heating the bath to temperatures approximating 1800 C.,- turbulently stirring the said bath to thoroughly and uniformly distribute the said oxide throughout the said bath, circulating a reducing gas comprised of a mixture of hydrogen, carbon monoxide and nitrogen over the surface of the bath until the oxide content is reduced to relatively low percentage, replacing the said reducing gas with substantially pure dry hydrogen, heating the bath to a temperature of about 2000 C.,. and maintaining this said temperature for a time interval required to. obtain substantially complete reduction of said oxides, the said bath throughout the circulation of said gases being maintained in said state of turbulent stirring.

12. The method of manufacturing iron-chromium alloys comprising forming a molten metal bath comprised of low carbon iron, superposing chromium oxide on the surface of said bath, heating the bath to temperatures approximating 1800 C., circulating a reducing gas comprised in major part of hydrogen and in minor part of other reducing and inert gases over the surface of the bath, turbulently stirring the bath to thoroughly disperse the said oxide throughout the said bath and to maintain a substantially uniform bath composition and to increase the eifective surface area of said bath in contact with said gas, continuing the circulation of said gas until the oxide content has been reduced to relative low percentage, replacing the said reducing gas with substantially pure dryhydrogen, increasing the temperature of the bath to a maximum of about 2000 0., and maintaining this said maximum temperature for a time interval required to obtain substantially complete reduction of the remainder of said oxides, the turbulent stirring of said bath being continued throughout the circulating of said I hydrogen over the surface thereof.

in of replacing the final reducing gas atmospiiere with an oxidizing atmosphere.

15. In the manufacture of iron-chromium alloys in accordance with the method of claim 1,

the steps of forming the molten bath of a low carbon iron alloy, super-posing chromium oxide thereon, heating the bath to temperatures at least approximating 1800" C., and circulating over the surface of the bath a reducing gas comprised of a mixture of hydrogen, carbon monoxide and nitrogen, the hydrogen content thereof being predominant and said gas containing aproportion of gaseous hydrocarbons in an amount substantially neutralizing any oxidizing gases and molature contained in said mixture, and further add- 10 ing such an excess of hydrocarbon vapors that clouding of the atmosphere of the furnace by precipitation of soot is just avoided.

- ROHN. 

